I have to say I fought with it for a week as well just to get ours ported from 3.x to 4.
Finally I figured out you could pass a Node to the load method to get nodes loaded.
Some how I managed to miss that in documentation and all searching on line.

Now what I wish I could figure out quickly is how to on ItemClick, to not only load the nodes, but also expand it after the load happens. Anyone got that down?